ground-based ftir update caviar meeting npl, 29 th september 2010 tom gardiner, marc coleman
TRANSCRIPT
Ground-based FTIR Update
CAVIAR MeetingNPL, 29th September 2010
Tom Gardiner, Marc Coleman
Monday 10 April 2023
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Summary
• Aged mirror reflectivity measurements• ‘Smoothed’ calibration function• CAVIAR blackbody – uncertainty
assessment and paper preparation
Monday 10 April 2023
3
Aged mirror reflectivity
• Absolute reflectivity measurements of gold mirror used in solar tracker during Jungfrau campaign now underway.
• Measurements under ambient conditions just completed.
• Measurements under different humidity conditions underway.
Monday 10 April 2023
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Updated reflectivity results
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3500 4000 4500 5000 5500 6000 6500
Wavenumber / cm -1
Ref
lect
ance
/ %
Unused low Unused high Cam low Cam high Jungf low
Monday 10 April 2023
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Updated calibration function
• Latest mirror reflectivity measurements have confirmed transmission properties for both field campaigns and related calibration exercises.
• Extrapolation outside of reflectivity measurement region has been revisited, and a (hopefully) improved estimate derived.
• Updated calibrations functions now available that include these revisions, and integrate solar limb darkening effects.
Monday 10 April 2023
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Raw Calibration Function (Phase 1 – broadband InSb measurement)
Monday 10 April 2023
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Issues with raw function
• Calibration function should be a smoothly varying function across the the spectral range.
• Raw function contains lots of high resolution information relating to laboratory water vapour and instrumental noise. This high frequency noise will feed into field results analysis.
• Particularly the case in the interesting high frequency region
• Raw function also not suited to re-sampling at different resolutions.
• Smooth underlying function is visually obvious, but need an analytical method to extract it.
Monday 10 April 2023
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Smoothing / fitting procedure
• Filter raw spectrum using repeated application of a sharp gradient exclusion filter.
• This removes the effects of the water lines and some high frequency noise.
• Fit a high order polynominal to the remaining points.
• This gives an analytical expression for the shape of the calibration curve, that can be applied to any frequency scale within the frequency limits of the original data.
• Should be particularly useful when ratioing two calibration curves to determine scaling functions for different gains / apertures.
Monday 10 April 2023
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Polynominal fit to smoothed calibration curve
Monday 10 April 2023
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Residual between smoothed calibration curve and original function
Monday 10 April 2023
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CAVIAR Blackbody
• Now have a write up of the blackbody uncertainties.• Covers the various sources of uncertainty including :
– Nextel coating reflectance– Cavity emissivity– Ambient reflection– Uniformity– Themometry– Cavity thermal gradient– Thermal gradient across coating– Formal traceability to the SI through AMBER facility
• This will form the basis for calibration paper.
Monday 10 April 2023
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CAVIAR Blackbody Uncertainty Table
Source Comment Condition Uncert. Units divisor dT (K)Nextal reflectance Accounted for in following section 0.5 % REmissivity and ambient reflection Accounts for uncertainty in emissivity modelling Blackbody @ 303K 0.005 K 1.73 0.003
(Use highest value) Blackbody @ 293K 0 KBlackbody @ 283K 0.002 KBlackbody @ 278K 0.002 K
Uniformity Blackbody @ 303K 0.01 K 1.73 0.006PRT Calibration 0.012 K 2 0.006PRT measurement by DMM 0.014 % 2 0.018Conductive heat loss along PRT Accounted for in following sectionDifference between PRTs Typically <20mK @ 303K, and <10 mK between
278K and 293K 0.01 K 1 0.01Thermal gradient across coating Assume coeff = 25 Blackbody @ 303K 0.0045 K 1.73 0.003Thermal gradient along cylinder 0.05% per K gradient; assume 0.5K gradient 0.015 K 1.73 0.009
Combined Standard Uncertainty (k=1) 0.024
Monday 10 April 2023
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International Intercomparison• CAVIAR blackbody used in first part of CEOS comparison of IR
brightness temperature measurements in support of satellite validation : Laboratory and Ocean surface temperature comparison of radiation thermometers.
• Took part in two stages– Stage 1 took place at NPL and involved laboratory
measurements of participants’ blackbodies calibrated using the NPL reference transfer radiometer (AMBER), while participants’ radiometers were calibrated using the CAVIAR blackbody.
– Stage 2 took place at Rosenstiel School of Marine and Atmospheric Science (RSMAS) and involved laboratory measurements of participants’ blackbodies calibrated using the NIST Thermal-Infrared Transfer radiometer (TXR), while participants’ radiometers were calibrated using the RSMAS and NIST water bath blackbodies.